Earthquakes are one of the costliest natural disasters in terms of both economic damages and loss of human life. In 2014, damages from earthquakes cost the global economy $313 million, and dozens of recorded earthquakes have produced death tolls that topped 100,000 people. These heavy penalties make earthquake prediction a critical area of research for seismologists. Previous research suggested that disturbances in Earth’s atmosphere may precede earthquake events, providing scientists with a measurable indicator of impending tremors. However, Masci and Thomas argue that the evidence for such a measurable precursor is lacking.

In the early morning hours of 16 October 1999, a 7.1 magnitude earthquake struck southern California approximately 22.5 kilometers from the Hector Mine quarry. Years later, scientists suggested they had evidence that it could have been predicted: In 2007, Pulinets et al. (Adv. Space Res., 39, 970–977, 2013, doi:10.1016/j.asr.2006.04.032) introduced what is now known as the spatial scintillation index, which uses GPS receivers to measure changes in the total electron content (TEC) in Earth’s ionosphere above faults. The authors stated that they had detected disturbances in the ionosphere before major earthquakes, including the 1999 Hector Mine earthquake. However, Masci and Thomas point out that research using the Kp index—an accepted indicator of geomagnetic disturbances—has linked the variation in TEC that Pulinets et al. detected to solar and geomagnetic activity rather than seismic events.

In a 2014 follow-up from Pulinets and Davidenko (Adv. Space Res., 53(5), 709–723, 2014, doi:10.1016/j.asr.2013.12.035) defending the index, the researchers used the Dst index—a measure of magnetic storm strength—to show that TEC variability was greater the day of the earthquake than on the days after, despite the fact that the Dst index showed that geomagnetic conditions were similar. Such a finding would indicate that the variability may be due to the imminent seismic event rather than changes in solar activity. However, Masci and Thomas state that the improper use of the Dst index—checking geomagnetic conditions on a global scale—led them to unfounded conclusions.

Further, the relationship between the spatial scintillation index and geomagnetic activity is varied—occasionally positive, sometimes negative—and at other times, no clear relationship is evident. As such, the authors conclude that the spatial scintillation index is not a reliable tool for earthquake prediction, nor can it be regarded as a reliable indicator for the geomagnetic conditions. (Radio Science, doi:10.1002/2015RS005734, 2015)

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